During the initialSync collection copy phase, all the output is going to be inserts. For Merge sinks, we can add an optimization where during this phase only, we use the `collection::insert_many` method to insert these docs. The result of this method indicates on a per document basis whether the insert was successful or not. If some inserts failed, then for these failed inserts, we will create a batch of all the failed inserts and then fall back to the default merge code for this batch.

If we are not in collection copy phase, we will simply fall back to the default merge code.